In the case of capacitive fill level measurement or monitoring, a probe unit (for example, a probe rod or a probe cable) and the wall of the container, in which the medium is located, or a second probe unit, form the two electrodes of a capacitor. The medium serves, in such case, as the dielectric. Since the capacitance of the capacitor changes as a function of fill level height, fill level can be deduced from the capacitance. For measuring capacitance, in such case, an actuating signal is fed to the probe unit. This is most often an electrical, alternating voltage of predetermined frequency. Tapped from the “measuring capacitor” is a measurement signal. This is usually an electrical current signal, which is, most often, converted, for example by a resistance element, into a voltage signal for further processing. From this, then the capacitance, and, therewith, the fill level, is ascertained or monitored.
Since a conductive medium can lead to a short circuit or, at least, to a corruption of the measured values, in practice, completely insulated probes are used. In such case, for example, a plastic insulation (PP, PTFE, PFA) is used. In the case of conductive media, in this way, a measurement independent of the dielectric constant is achieved, in which only the insulation capacitance covered by the medium is still relevant.
In practice, for example, in the case of measuring heated water, it can happen that diffusion of the medium through the probe insulation will occur and that, in this way, ohmically conducting, transition, or interface, resistances form in the process connection. In the case of diffusion, an electrical connection can occur with the container wall or with ground, for example through the medium, especially in the region of the—most often grounded—housing. A further possibility is that, through damage to the insulation, the medium can penetrate to the probe unit and form, in this way, likewise, an ohmically conductive resistor parallel to the measuring capacitance. This effect of additional resistance leads to a corruption of the measured value and even to total failure of the measuring device.